Josselin Castle (, , ) is a medieval castle at Josselin, in the Morbihan department of Brittany, France, first built in 1008 by Guéthénoc, viscount of Porhoët. The town and castle were named after Guéthénoc's son, Goscelinus, and rebuilt at various times since. The current castle was built by Olivier V de Clisson after 1370. He had acquired the land as part of the dowry on his marriage to Margaret of Rohan. It has been designated as a monument historique since 1928. History (or Guithenoc), vicomte of Porhoët, Rohan and Guéméné, began to build the first castle on the site around the year 1008, choosing a rocky promontory overlooking the valley of the Oust. The new fortress was named after Guéthénoc's son, (or Josselin). The name is recorded in the Cartulary of Redon Abbey (1080) as castellum et castrum Goscelini, but already by 1108 it was appearing as Castellum Joscelini. The site chosen for the castle was excellent from both military and commercial points of view. Since the 9th century, there had also existed an annual pilgrimage in September to the (Notre-Dame du Roncier), which added greatly to the wealth of the lords and people of Josselin. In 1154, Odo II, Viscount of Porhoët, step-father, guardian and regent of the young Conan IV, Duke of Brittany, collected the Breton lords to deprive Conan of his inheritance, but was defeated by Henry II of England, who was also Count of Anjou, whose protection Conan had sought. Henry married his fourth son, Geoffrey, to Conan's only child, Constance, Duchess of Brittany, and Henry and his son pulled Josselin Castle down in 1168 and 1175. Henry II himself led the demolition and sowed salt into the ruins. During the Breton War of Succession (1341–1364), the garrison of Josselin fought the defenders of the nearby Castle of Ploërmel without any clear outcome. To break the impasse, the Battle of the Thirty was arranged, contested by thirty knights from each side, and took place on 26 March 1351 halfway between the two places. The men of Josselin defeated the champions of Ploërmel, who were four Bretons, six Germans and twenty Englishmen. In 1370 the Breton soldier Olivier V de Clisson (1336–1407), later Constable of France, acquired the lordship of Josselin and built an imposing new fortress with eight towers and a keep one hundred yards across. He married his daughter Beatrice to , heir to